The present invention relates to a method and apparatus for image encoding. More specifically, it relates to a method and apparatus employing a wavelet transform technique for image encoding.
Image compression methods using the discrete cosine transform have been standardized by the Joint Photographic Experts Group (JPEG) and Moving Picture Experts Group (MPEG).
Recently, however, there has been much interest in alternative image compression methods that use the wavelet transform technique. The wavelet transform iteratively separates low spatial frequencies from high spatial frequencies, to produce a multiresolution representation of an image. For many images, the wavelet transform offers the possibility of a higher compression ratio than the discrete cosine transform.
For example, Stephane Mallat, xe2x80x9cA Theory for Multiresolution Signal Decomposition: The Wavelet Representationxe2x80x9d, IEEE Trans. PAMI, Vol. 11, November 7, July 1989. pp 674-693 discloses a method for wavelet transform of images.
A conventional multi-level multi-dimensional wavelet filter for wavelet transform is constructed by connecting a plurality of base wavelet transformers in cascade. The base wavelet transformer performs a single-level one-dimensional wavelet transform. The base wavelet transformer is comprised of a single-level one-dimensional wavelet filter.
A problem is that the conventional method works only with rectangular images. If an image is not rectangular, the image must be embedded in a rectangle, and the non-image parts of the rectangle must be filled in with, for example, the average value of the image signal, or with signal values copied from the border of the image.
However, the compression efficiency of the resulting rectangular image tends to be impaired. This is because a large gap may occur between the image and the filled-in area, and more pixels than the original image area need to be encoded and transmitted. As a result, a large number of bits are required to encode the image.
Accordingly, it is an object of the present invention to execute a wavelet transform on an inputted image with an arbitrary shape.
It is another object of the present invention to provide a wavelet transform on an inputted image with high coding efficiency.
According to a first aspect of the present invention, a shape adaptive wavelet transform is executed on inputted images. First, a shape information of the image is obtained, the shape information containing at least a starting point and an ending point of a consequent image area of the image. Then, the image is filtered and divided into a plurality of resolutions, and each resolution is sub-sampled to obtain a sub-sampled signal. The shape information is converted to a new shape information, which indicates positions at which the sub-sampled image signal exists. Thereby, a one-dimensional wavelet transform on each line of the image is performed.
According to a second aspect of the present invention, an N-level shape adaptive wavelet transform is executed on inputted images. To the wavelet transformer of the invention, the image signal and shape information thereof are inputted. The first stage wavelet transformer generates first and second sampled signals and feeds them into next stages. The second and third stages execute wavelet transforms on the first and second sampled signals, respectively.